1. Field of the Invention
The present invention relates to an optical amplification fiber capable of amplifying light, an optical amplification module including the optical amplification fiber, an optical communication system including the optical amplification module, and an optical amplification method using the optical amplification module.
2. Related Background Art
The required transmission capacity of optical communication systems is increasing, so as to inevitably lead to increase in the number of signal channels in the Wavelength Division Multiplexing (WDM) optical transmission which is the mainstream at present. However, the increase in the number of channels without change of the conventional channel spacing (wavelength spacing) will result in expanding a necessary signal wavelength bandwidth and it can exceed the optical amplification band of Er-doped fiber amplifiers (EDFAs) as the optical amplification technology available at present. On the other hand, a seek for higher density of signal light by halving the channel spacing from 100 GHz of the conventional ITU grid to 50 GHz and further to 25 GHz is a desired approach from the viewpoint of increasing the transmission capacity while keeping the conventional width of the optical amplification band.
For achieving a high density of signal light, however, there is concern about degradation of signal quality due to enhancement of nonlinear interaction between signal channels as typified by four wave mixing (FWM). Efficiency η of generation of a conjugate wave in FWM is approximated by Formula (1) below and, where the channel spacing Δλ is halved, the generation efficiency η is expected to be up to sixteen times (12 dB on a decibel basis).
                    η        ∝                              1                                                                                α                    2                                    ⁡                                      (                    MFD                    )                                                  4                            ⁢                                                                    D                    2                                    ⁡                                      (                    Δλ                    )                                                  4                                              .                                    (        1        )            
In the formula α represents an absorption coefficient in the case of fibers for transmission, or an unsaturated absorption peak per unit length in the case of fibers for optical amplification. For example, in the case of an Er-doped optical fiber (EDF), α is determined by an Er dopant concentration, an overlap between an Er-doped region and a signal light mode field, and an absorption cross section of Er ions determined by a glass composition. Furthermore, MFD (Mode Field Diameter) is a mode field diameter for the fundamental mode of signal light, D chromatic dispersion, and Δλ the channel spacing. Since only Δλ is a parameter dependent on setting on the signal light source side, η0 represented by Formula (2) below is introduced for convenience sake of purely comparing characteristics of EDF.
                              η          0                =                              1                                                                                α                    2                                    ⁡                                      (                    MFD                    )                                                  4                            ⁢                              D                2                                              .                                    (        2        )            
Only MFD has the same order as Δλ in above Formula (1) (or above Formula (2)), and expansion of MFD is the most effective in decreasing η. It is pointed out that this phenomenon occurs not only in the transmission fibers, but also inside optical fiber amplifiers such as EDFAs (Erbium-Doped Fiber Amplifiers) used as repeaters. This tendency is prominent, particularly, in L-band EDFAs necessitating a long EDF.
For decreasing η in above Formula (1) (or η0 in above Formula (2)), the simplest technique independent of the other parameters is to increase the Er concentration as in Embodiment 1 or 3 of Document 1 (Japanese Patent Application Laid-Open No. 2000-31571). However, too high Er concentrations actualize a phenomenon in which Er ions exchange energy with each other to become not engaged in optical amplification (so called concentration extinction), so as to decrease pumping efficiency.
Other proposed techniques for decreasing η in above Formula (1) (or η0 in above Formula (2)) include increase of the core diameter, expansion of the outer diameter of the Er-doped region, expansion of MFD, design of the refractive index profile to increase chromatic dispersion, and so on as suggested in Document 1 or in Document 2 (Japanese Patent No. 3228374).
In addition, Document 3 (M. E. Fermann, “Single-mode excitation of multimode fibers with ultrafast pulses,” Optics Letters, Vol. 23, No. 1, p. 52, 1998) discloses the technology using a mode converter comprised of a complex lens system for input of single-mode signal light into a multimode optical amplification fiber. The propagatable length of the fundamental-mode light in this multimode optical amplification fiber is set to the length where the coupling efficiency to a higher-order mode is 1/e2, and the propagatable length is approximately 3 m even under such less-restricted standard. The propagatable length of the fundamental-mode light in the multimode optical amplification fiber as described in Document 4 (U.S. Pat. No. 5,818,630) is at most 1.1 m.